This invention relates to apparatus for use in lamp making and, in particular, to apparatus for cross-rolling a lead wire to form a flattened portion for sealing in a vitreous bulb.
In sealing a metallic conductor or lead wire through quartz or quartz-like glass, the high temperatures necessary for fusing quartz limits, for practical purposes, the choice of wire to molybdenum or tungsten. Both of these metals have coefficients of expansion much greater than that of quartz or quartz-like glass. For example, molybdenum has a coefficient of expansion approximately ten times that of quartz. It is thus necessary to shape the wire so that the wire remains sealed to the quartz upon cooling. This is achieved by shaping a portion of the wire to increase the surface area thereof, typically in the form of a foil of approximately one-mil thickness. The foil portion of the lead bonds to the quartz and, upon cooling, undergoes tension but will not crack the quartz.
In the past, the foil has been formed by either longitudinal or cross-rolling the wire. In U.S. Pat. No. 3,102,443, for example, apparatus is described for cross-rolling the wire between two pivoting dies. A difficulty with this system is that the wire is rolled one segment at a time and requires pre-rolling treatment (flattening) to assure that the wire does not pop out from between the dies. Also, since the wire is supported by the dies, care must be taken to assure that the wire does not leave the pinch area and move out of position.
It is therefore desirable to have foil forming apparatus capable of operating on more than one wire at a time. Also, it is desirable to have the foil forming apparatus operate on round wire. In addition, it is desirable to have the foil formed with an elliptical or tapered cross section, feathered at the edges, to assure that the seal is not broken in this area when the quartz cools.